Cutting of cementitious materials

ABSTRACT

A method and apparatus for cutting thick sections of cement-based materials, the method comprising mutually traversing a surface to be cut with a laser beam at a power density sufficient to produce a depth of molten material having a maximum depth of 10 mm at each traverse; allowing the molten material to solidify; breaking the solidified material into particles; and removing the particles by suction means.

This invention relates to a method and apparatus for thick-sectionconcrete cutting, e.g. up to 1 m and deeper, particularly, though notexclusively to concrete that has contaminants embedded in thesub-surface matrix, and more particularly, though not exclusively, toconcrete that is contaminated by radio-nuclides, where any materialwhich is removed during cutting requires stringent containment.

Nuclear reactors and nuclear processing facilities in general haveservice lives of approximately 40 years. As part of the decommissioningprocess the reactor has to be dismantled, and the concrete wall, whichmay be over 1 m in thickness, that served as a biological shield, has tobe broken down. After reactor shutdown, the concrete still containssignificant amounts of residual radiation. Common radioactivecontaminants are strontium-90, caesium-137, and cobalt-60. During thedismantling process it is imperative that these radio-nuclides are notreleased into the atmosphere, and that the exposure of site personnel tothese substances is kept to an absolute minimum. Conventional techniquesfor cutting concrete, such as diamond blade and diamond wire sawing,diamond core drilling inclusive of stitch drilling, water jet cutting,and thermite lance cutting, all create substantial amounts of effluentin the form of waste water, dust and fumes which also have to becontained and collected and themselves form part of the volume of wastewhich has to be treated and stored. Some of these prior art techniqueshave access difficulties since they require access from both sides ofthe concrete structure, e.g. diamond wire sawing. They are thus notideally suitable for this particular application.

Prior art laser-based concrete cutting methods include both single- andmulti-pass techniques. In general the most important aspect thatcontrols the depth of cut, is the efficiency with which the moltenmaterial can be removed. In the case of single-pass techniques, a holeis typically first drilled mechanically through the concrete after whichthe beam is traversed across the segment to be cut and the moltenmaterial is ejected to the opposite end by the pressure of an assistgas. However, the use of an assist gas brings further difficulties inthat it has the effect of cooling down the molten concrete which isalready very viscous and difficult to remove thus exacerbating theproblem. There are also problems with maintaining the focus of a gas jetin air over deep cutting distances. The focal plane of the laser beamcan be placed either at the concrete surface, or below it according tothe preference of the operator. However, neither strategy is ideal whenattempting to cut a very large thickness in one pass.

Single-pass methods proposed for enhancing efficiency include: usinghigh-pressure gas for assisting the release of molten dross;introduction of explosive powder into the kerf to blow out the moltenmaterial; shooting explosive bullets into the kerf and triggering thesame using heat generated by the laser beam; enhancing the laser powerdensity by focusing three laser beams on a single spot and blowing themelt out laterally and downwards; introduction of eutectic formers todecrease the fusion temperature of the concrete; and, injection ofhigh-pressure water to cool and crush the molten concrete. Even at powerlevels as high as 15 kW, these techniques have yet to demonstrate thatthey can penetrate deeper than 180 mm, and as such are not suitable fordeep-section cutting.

For multi-pass strategies the beam is normally focussed on the cuttingsurface and the molten material is either ejected towards the enteringlaser beam by an assist gas or allowed to vitrify and removedsubsequently. In JP-A-63157778 the laser beam is focused on the surfaceto be cut and the maximum depth of cut is sought, typically of greaterthan 45 mm or more using a laser of about 5 kW or more output. Aftersolidification the molten concrete is removed by various mechanical orchemical techniques, and the process is repeated by re-focusing thelaser beam on the new surface at the base of the previously treatedtrack. However, problems can arise if the solidified material becomestoo thick, since it is effectively glass-like, and in solid, thickpieces can be difficult to remove by rotary brushes, blades and thelike. In the case of cutting blades there is a practical limit to thedepth to which these can be used.

JP-A-62181898 describes a technique of multi-pass treatment by shootingexplosive bullets directly into the melt to eject the molten concreteand to induce local fragmentation of the surrounding solid concrete.This method is clearly dangerous and also has the additionaldisadvantages of adding to the waste stream and of potentially spreadingthe contaminated material over a large area, impeding easy retention andcollection thereof.

Whilst these prior art methods can in principle go to much greaterdepths of cut, there are definite limits on the depth achievable byrotary tools and the achievable geometric complexity. In general though,the release of effluent into the atmosphere is difficult to control and,as such, existing technologies are not ideally suited for cuttingcontaminated material.

A further disadvantage of the prior art methods which tend to try toproduce as great a cutting depth as possible is that as a result of themuch greater heat input and temperatures reached, the generation ofexcessive vapour, which may contain relatively large amounts ofradioactive species, is correspondingly high and potentially dangerousto people and to the environment and the vapour is not easily contained.

An objective of the present invention is to provide a method andapparatus for effecting deep-section cutting of plain and reinforcedcontaminated concrete, which allows for easy management of generatedwaste.

According to a first aspect of the present invention there is provided amethod for the cutting of thick sections of cement-based materials, themethod comprising the steps of: mutually traversing a surface to be cutwith a laser beam at a power density so as to produce a depth of moltenmaterial of a maximum of 10 mm at each traverse; allowing said moltenmaterial to solidify; breaking said solidified material into smallparticles; and, removing said small particles by suction means.

According to a second aspect of the present invention apparatus for thecutting of thick sections of cement-based materials comprises: means formutually traversing a surface to be cut with a laser beam at a powerdensity so as to produce a depth of molten material of a maximum of 10mm at each traverse; means for breaking melted and re-solidifiedmaterial into particles; and, means for removing said particles bysuction means.

In this specification the term “thick sections” is intended to meandepths of concrete or chemically similar or analogous materials of theorder of 1 m or more. However, it should be borne in mind that a methodcapable of cutting such thicknesses must also be capable of cutting muchthinner sections and thus, this term is not to be taken as a limitation.

In this specification the term “cement-based” is intended to cover allcommon building materials including, for example, Portland cement,concrete having a substantial second phase of aggregate material (theaggregate may be of any type of sand or stone) and a cement matrix andnatural stone materials.

Whilst the present invention was developed for cutting contaminatedconcrete the invention has wider application in general civil andstructural engineering where un-contaminated concrete is involved.

Unlike prior art laser-based techniques, this is accomplished by usingrelatively low laser power densities. The low cut depth per pass meansthat the degree of heat input is correspondingly less in total and thatthe degree of vapour formed is lower than in the prior art. Furthermore,the material is melted relatively quickly and because the ratio ofvolume of melted material to volume of surrounding unmelted material isrelatively low, the melted material solidifies quickly and is of agenerally weak and porous nature which is easily broken up and removed.

The laser beam may be either defocused, optically parallelised, orpreferably, a raw (unfocused, parallel) laser beam may be used. Morepreferably, the laser beam may be unfocused and parallel but also ofsubstantially rectangular cross sectional shape. The focused laser beamsused in the prior art, whilst providing high power densities at thepoint of impingement on the surface, are unable to cut to any greatdepth because of the conical nature of the beam and the consequenttapering nature of the cut channel which they form. Parallel beams onthe other hand are able to cut to much greater depths. Nevertheless,parallel beams of circular cross section also have apparent limitationsin that they also tend to produce a tapering cut channel but the slopeof the sides of the cut channel is much less than with focussed beams.The tapering cut produced with circular beams is due to the powerdensity in the beam spot being of a psuedo-Gaussian nature such that thepower density at the edges of spot are of lower power density thantowards the beam centre line in the direction of movement. Furthermore,with a circular laser beam when the beam is being traversed across asurface, the material at the beam spot lateral edges is subjected to asignificantly lower power density for a shorter time than that materialinwardly of the beam edges towards the centre. However, even though thekerf width initially tapers quite rapidly, it appears to reach a widthwhere deeper cutting width remains constant.

A rectangular section laser beam such as a square laser beam, forexample, overcomes this disadvantage in that when the beam is beingtraversed across the surface all of the material falling within the spotare at least treated for the same time since the beam spot forms anadvancing planar front on the material and beam depth in the directionof travel is also constant.

The solidified material inside the kerf (the “kerf” being the term usedfor the material removed in the cut or the cut opening) may be broken upby percussive and/or compressive treatment such as by hammering and/orabrasion, for example.

The maximum depth of melted material is 10 mm per pass.

Preferably, however, the thickness of the material melted in each passmay lie in the range from about 0.5 to about 5 mm per pass. Morepreferably, the thickness may be about 1 to 4 mm per pass. Even morepreferably the melted thickness may be about 1 to 2 mm per pass.

The thickness of the molten material is preferably typically kept to afew mm only per pass, in which state minimum force is required fordislodging and crushing the treated material, using a vibratingmechanical device, for example, into pieces small enough to be easilysuctioned off and transferred to a filter system by pneumatic transport.By melting only a few millimetres of material per pass the quantity ofheat input is minimised and the molten material re-solidifies veryquickly and in a very porous and weak condition due partly toout-gassing of relatively volatile constituents such as water ofcrystallisation, for example. Where the molten layer is too thick, thequantity of heat input is greatly increased and the time forre-solidification of the molten material consequently also increasesresulting in the solidified material being more homogeneous, inherentlymuch stronger and consequently more difficult to break up. The kerf maybe broken up by a vibrating mechanical, device which may also have aconduit associated therewith so that the broken particles may be removedby suction. It has been found that with the method and apparatusaccording to the present invention the great majority of the crushedmaterial comprises a particle size of less than 2 mm which is easilyremoved by suction means.

The crushing and extraction device may be inserted into the cut kerf andfollow the movement of the laser beam at a distance sufficient to allowsolidification before contact is made.

Pressure exerted on the solidified material to effect crushing is amaximum of about 100 MPa, thus making the process easy.

A mechanical vibrator powering a tubular crusher device, and whoseheight can be appropriately adjusted may be used. Typically the tip ofthe crusher is a tubular hardened tool tip appropriately shaped tomaximise pressure such as by the inclusion of teeth, for example, at itsimpacting or cutting end. The crusher can oscillate axially, or rotateor embody both these motions. The diameter of the crushing device shouldbe less than the diameter of the laser beam, i.e. less than the kerfwidth.

Generally, the concrete matrix may be composed of Portland cement andaggregate. The aggregate is typically sand (˜80% SiO₂), limestone(CaCO₃), basalt, granite, or andesite, with particle diameters rangingfrom less than 1 mm to over 10 mm. Dry Portland cement in turn ispredominantly composed of finely divided SiO₂ and CaO, with smallerquantities of Fe₂O₃, Al₂O₃, and MgO. Upon hydration it forms a complexgel-like structure that contains water bound in the various crystallinephases present as well as free water adsorbed in the complex porestructure of the material. Below an energy-density threshold of about100 W·cm⁻² to 200 W·cm⁻², differential thermal expansion and/ordehydrated water vapour cause surface layers to dislodge, without anymelting taking place, in a process known as “scabbling”. Above thisthreshold, melting takes place. The exact values of the energy densityrequired for melting and the fusion temperature vary with the qualityand composition of the concrete, but in general these are of the orderof 300 W·cm⁻² and 1000° C. respectively. Useful laser power densitieshave been determined to be between about 300 and about 12000 W·cm⁻².

As the depth of the kerf increases and its width narrows slightly,compensatory changes by increasing power density may be made so as tokeep the width of the kerf sufficient to permit access for dross removalmeans.

The outer diameter of a circular laser beam at the point of interactionwith the concrete may lie in the range 8-30 mm. Rectangular or otherbeam shapes are also viable. Temperatures of around 1000° C., just abovethe melting point of concrete, can be used. The whole temperature rangeup to the boiling point at 2400° C. is useable if associated fumeextraction facilities are strong enough. The concrete temperature andthe traverse velocity determine the vapour-to-melt ratio. The workableregion for this ratio is between 0.05 and 3. However, in order tominimise fume and vapour generation it is desirable to work at the lowerend of the range closer to the melting temperature of the concrete.

The process parameters of prime importance for cutting are: beamtraverse speed; laser energy; and, beam spot diameter or area (powerdensity being derived from the latter two values). For this method thegas flow rate is less important when concrete is being cut, the gasessentially being used only to ensure the cleanliness of the opticalcomponents by preventing fume and debris from reaching them. Duringtreatment both melting and vaporisation take place. In general both thevapour-to-melt ratio and the depth of cut per pass decrease as powerdensity decreases, and as traverse speed increases. The chosen regime isthus a trade-off between the amount of vapour the user is prepared tocontend with, and the minimum cutting speed desired. The larger the beamspot the higher the efficiency of the energy usage. A beam diameter orwidth at the work surface of about 8-10 mm minimum is preferablyrequired to ensure a kerf width wide enough for optimal entry ofpercussion crushing and suction tooling. The beam spot is surrounded bya heat-affected zone (HAZ), the area and crushability of which arefunctions of the operating parameters used. Because of the presence of aHAZ and the movement of the crusher, the kerf width is generallyslightly wider than the laser beam diameter.

In one embodiment of the present invention the laser beam may be movedacross the work surface at a traverse velocity of between 3cm·min⁻¹ and40cm·min⁻¹.

The material removal rates vary according to the type of laser. For thediode laser, a mass removal rate of the order of 150 cm⁻³·kWh⁻¹ may beachieved and about 100 cm⁻³·kWh⁻¹ for a CO₂ laser.

The beam may pass through a suction cup, coupled to a HEPA filter andextraction system. The mouth of the suction cup may be positioned asclose as possible to the top of the kerf in such a way as to extract anyvapour evolved during the process. The cup may be mounted on arobot-driven assembly that houses either the laser oscillator itself, orthe components required for focusing the laser output from the outlet ofa fibre optic cable, or other beam delivery unit. Typically, the rawbeam from a CO₂ or diode laser may be used. However, Nd:YAG, a Fibrelaser or chemical oxygen-iodine laser (COIL) can also be used.

Where appropriate the laser system may embody a beam collimator such asa two-lens system for the manipulation of power density to form part ofthe system of power density control. This enables the beam diameter tobe adjusted to a desired size whilst still maintaining beam parallelism.

A diode laser is small and compact and the laser oscillator may form apart of a mobile apparatus. This is particularly relevant as therectangular form of the laser beam may be retained and which wouldotherwise be lost if the laser beam is transmitted by fibre optic cable.

Concrete structures are frequently reinforced with metal bars such assteel bars running through the concrete. When such concrete structuresare being broken down it is necessary also to cut through the steelbars. In this regard it is advantageous to employ a laser device whichhas a reserve of power over that required to cut the concrete accordingto the method of the present invention due to the higher energy densityrequired for cutting the metal, caused by its greater thermalconductivity. A fibre optic monitoring device may be inserted into thekerf to detect the presence of steel bars, and for monitoring theprocess. Desirably, there should be an oxygen supply able to provide asuper-stoichiometric oxygen atmosphere to make full use of the enthalpyof reaction for the oxidation of steel. The metal waste generated may bein the form of relatively finely divided ferric oxide powder or piecesof re-solidified molten steel. Apparatus for carrying out the method ofthe present invention should desirably include a conduit for supplyingoxygen and having an outlet which is positionable as closely as possibleto the area of impingement of the laser beam. The aforementioned minimumkerf width makes this readily achievable. Because of the higher thermalconductivity of steel, a higher energy density is required to keep theoxygen-steel reaction above its ignition temperature. This value istypically above 1500 W·cm⁻² for a 15 mm OD steel bar depending ontraverse speed. If a lower-power laser is used this can easily beachieved by adjustment of the position of the focal plane. If reinforcedconcrete is to be cut using a raw laser beam, the energy density of thesaid beam will have to exceed this value, or a value scaled to thediameter of the steel bars in question. An alternative to laser cuttingof the steel bars, is using a flame, e.g. oxy-acetylene, and to use theoxygen delivery system for his purpose.

The laser beam may be directed at the concrete via a hollow suctionchamber or cup through which process-generated fumes may be extractedand through which may pass an oxygen delivery tube for directing anoxygen jet at the beam spot when cutting reinforcing steel bars. Apercussion-extraction tube may be mechanically coupled to a supportstructure holding the laser device and positioned a short distancebehind the beam spot, for removal of solidified dross. Both extractionstreams may pass through an absolute filter system in which radioactivecontaminants can be contained according to the needs of the user.

In order that the present invention may be more fully understood,examples will now be described for the purposes of illustration onlywith reference to the accompanying drawings, of which:

FIG. 1 shows a graph of penetration depth per pass vs pass number for aCO₂ and a diode laser;

FIGS. 2A and 2B show photographs of concrete slabs cut using a CO₂ laserand a diode laser, respectively;

FIG. 3 shows a graph of kerf width vs kerf depth of cut for a CO₂ laserand a diode laser;

FIG. 4 shows a graph of volume of concrete removed vs time for a CO₂laser;

FIG. 5 shows a representation of a cut concrete slab under conditionsdifferent from the cut slabs shown in FIG. 2;

FIG. 6 shows a schematic representation of apparatus according to thepresent invention for carrying out the method of the present invention;and

FIG. 7 which shows a histogram of particle size distribution in thecrushed kerf material after processing by the apparatus shownschematically in FIG. 6.

Experimental work was carried out using a Rofin-Sinar RS-1000 (tradename) 1.2 kW, fast axial flow CO₂ laser and a Laserline LD-160-1500(trade name) 1.5 kW high-power diode laser. For comparative work the CO₂laser yields a circular beam operating in the TEM₀₁ mode, at 10.6 μm,and was fitted with a 125 mm focal length lens. The diode laser yields arectangular section, multimode beam at 808 and 940 nm and was fittedwith a 300 mm focal length lens. The laser beams were traversed relativeto the concrete slabs at a speed of 2 m·s⁻¹. In both cases the beam wasdefocused to give a power density of 1000-1100 W·cm⁻². The distancebetween the lens and the concrete interaction surface was maintainedconstant. The diode beam shape was thus kept constant at a rectangularsection of dimensions 12 mm×8 mm (with the 12 mm dimension forming theadvancing front) and the CO₂ laser beam diameter at 11 mm (measured atthe bottom of the kerf). Gas flow rates were kept to a minimum to avoidcooling the molten dross and to protect the lens from fogging.

FIG. 1 shows a graph of kerf cut depth vs pass number. A kerf depth of120 mm was reached after 74 passes with the diode laser and after 94passes using the CO₂ laser. An average penetration rate of 1.6 and 1.3mm was achieved per pass, respectively. From FIG. 1 it is clear that cutkerf depth is substantially constant per pass and that the biggestinfluence on cut depth per pass is the composition of the concrete atthe relevant depth and whether or not limestone aggregate isencountered.

FIGS. 2A and 2B show sections of concrete cut by the CO₂ and diodelasers under the conditions specified in the preceding paragraph,respectively. It may be seen that the kerf cut with the diode laser inFIG. 2A is of much more uniform cross sectional shape than that cut withthe CO₂ laser. This difference in cross sectional shape, i.e. taperingmore with depth than with the diode kerf is accounted for by thevariation in power density across the advancing width of the laser beamas explained hereinabove.

FIG. 3 shows a graph depicting the variation in kerf width withincreasing depth and again shows the more uniform kerf width withincreasing depth of the diode laser. However, this improved uniformityof kerf width is a feature related to the rectangular cross sectionalshape and parallel form of the laser beam rather than the fact that itis a diode laser per se.

FIG. 4 depicts results obtained with plain concrete and the raw beam ofa CO₂ laser. Power density in this example was kept constant at a 550W·cm⁻² and a traverse speed of 120 mm·min⁻¹ was used. The penetrationrate is linear, however, the removal rate tapers off slightly due to anarrowing kerf. The narrowing kerf is due to wall losses and to thedifferential energy density across the circular laser beam as explainedhereinabove but is easily compensated for by an appropriate adjustmentof the power density. The depth of cut shown in FIG. 5 is 300 mm using a1 kW laser.

An additional feature which is exploitable when cutting a concretestructure such as a nuclear reactor housing during decommissioning andwhich is predominantly contaminated on the inside, is that access ispossible from the outside, and the laser beam does not have to penetratethe structure fully thus causing undesirable damage to objects on andbeyond the inside surface. The thermal stresses which develop during theprocess invariably cause cracks through the final few centimetres,making laser treatment there redundant. This phenomenon can be seen inFIGS. 2 and 5 where at the bottom of the cut there is clearly a crackextending through the remaining thickness.

FIG. 6 shows a schematic representation of apparatus 10 according to afirst embodiment of the present invention. A partially-cut work piece 12(shown only in part) is static, being part of a larger structure (notshown) being dismantled. The apparatus comprises a housing 14 (shown asan enclosure by a dashed line) which holds and supports in an operablemanner various apparatus items which are traversed over the surface 16of the work piece being cut. The fixture 14 is held in this embodimentby a multi-axis robotic arm (not shown) in known manner. The apparatusitems held and supported by the housing 14 include: a source 18 of laserlight 20; an extraction cup 22 in the form of a shroud which covers thepart of the cut surface in which the point of impingement 24 of thelaser beam is located; an oxygen conduit 26; and, a vibrating percussiontool 28 driven by a controllable vibrator/positioning device 30. Anextraction and filtering system 32 is connected to the cup 22 and to thepercussion tool 28, the extraction system including a cyclone 34 toremove coarser debris, an absolute filter 36 to remove fine particlesand, an extractor fan 38.

If a compact CO₂ or high-power diode laser is used, a laser oscillator18 is mounted and moved along with the aforementioned components. If,however, a Nd:YAG, diode, Fibre or COIL laser is used, the beam 20 canalternatively be delivered by a fibre optic cable and numeral 18represents an appropriate lens system.

Beam delivery via an appropriate reflecting mirror system (not shown) isalso possible.

In use, the housing 14 is traversed across the surface 16 by the robotarm (not shown) in the direction of the arrow 11 and the laser beampoint of impingement 24 causes the concrete to melt thereat and leavebehind a shallow track 40 of molten material which re-solidifies at 42.The percussion tool 28 in the form of a tube 44 tipped by a wearresistant material 46 follows behind the point of impingement of thelaser beam at distance where the molten concrete material hasre-solidified at 42 and breaks up the solidified material byreciprocating action generally in the direction of the tube axis and/orby rotation about the tube axis driven by the unit 30. The crushing tubeis fitted with a tip manufactured from a hard material such as tungstencarbide or titanium nitride coated tungsten carbide, for example, inorder to increase wear resistance, and shaped in such a fashion as tomaximise pressure at the points of contact between the tip and theconcrete dross. The solidified material breaks up very easily due to thelayer thickness being shallow, i.e. less than 5mm, and, because thematerial has melted quickly with moisture and volatile constituentout-gassing from the melted material itself and from the underlyingconcrete causing considerable porosity in the quickly re-solidifiedmaterial which consequently has little strength. In this embodiment thecrushing device also serves a second purpose of extracting the crushedmaterial through the tube 44 bore and delivering the debris to thefilter system 32 by means of suction provided by the extractor fan 38.The extractor cup 22 is sealed as far as is possible to the surface 16by seals comprising resilient rubber strips or brushes 48 so as toprevent egress of fume and debris.

The process is controlled in such a way as to keep the thickness of themolten zone 40 below a few mm to ensure that a minimum of pressure isrequired for crushing the dross, and that it can be accomplished in acontrollable way without excessively ebullient vaporisation during themelting phase of the process.

FIG. 7 is a histogram showing the particle size distribution of crushedkerf resulting from the method according to the present invention asdepicted in the samples of cut concrete slabs shown in FIGS. 2 and 5. Itmay be seen that the largest fraction number comprises particles of asize of 1-2 mm with the bulk of crushed material below this figure. Onlyabout 10% of crushed material has particles in the range 2-4 mm and 1-2%at a particle size over 4 mm. Thus, virtually all of the crushedmaterial may be removed by the apparatus shown in FIG. 6.

The kerf width is kept wide enough to allow easy access to thecrushing-extraction tube, and which makes depths of out of 1-2 mpossible.

Concrete is frequently reinforced with steel bars (not shown) whichrequire cutting if the larger structure is to be dismantled. An oxygensupply tube 26 is provided in order to supply a super-stoichiometricoxygen quantity to assist in cutting such reinforcing bars by enhancedoxidation potential. The positioning of the tip 50 of the tube 26 isaccomplished by a mechanical positioning device 52 as indicated in FIG.6.

Although in the above described embodiment the solidified slag layer isbroken up and removed by the combined crusher and suction tube 28, theseitems may be independently provided such as by a dedicated crushingmember and by a separate suction tube following the crushing member.Furthermore, more than one crushing tube/suction device may be providedto deal with remaining particles too large to be removed by the firstsuch device.

1-28. (canceled)
 29. A method for cutting thick sections of cement-basedmaterials, the method comprising: mutually traversing a surface to becut with a laser beam at a power density sufficient to produce a depthof molten material having a maximum depth of 10 mm at each traverse;allowing said molten material to solidify; breaking said solidifiedmaterial into particles; and removing said particles by suction means.30. A method according to claim 29 wherein a plurality-of traverses aremade along substantially the same cutting path.
 31. A method accordingto either claim 29 wherein the laser beam is unfocused.
 32. A methodaccording to claim 29 wherein the laser beam is a parallel beam.
 33. Amethod according to claim 29 wherein the laser beam has a rectangularcross section.
 34. A method according to claim 29 wherein the materialis removed directly after solidification after each pass.
 35. A methodaccording to claim 29 wherein the solidified material is broken up by ahollow crushing tube which also serves as a material extractor conduit.36. A method according to claim 29 wherein the depth of the moltenmaterial at each pass lies in the range from 0.5 to 5 mm.
 37. A methodaccording to claim 29 wherein the pressure required for crushing thesolidified material is less than 100 MPa.
 38. A method according toclaim 29 wherein the laser power density lies in the range 300 W·cm⁻² to3000 W·cm⁻².
 39. A method according to claim 29 wherein the beamtraverse speed lies between 3 cm·min⁻¹ and 30 cm·min⁻¹.
 40. A methodaccording to claim 29 wherein an oxygen jet is applied directly at thebeam spot when reinforcing steel bars are being cut.
 41. A methodaccording to claim 29 wherein the surface temperature of the materialbeing treated lies in the range 700° C. to 2400° C.
 42. A methodaccording to claim 29 wherein the vapor-to-melt ratio lies in the rangebetween 0.05 and
 3. 43. A method according to claim 29 wherein thematerial removal rate lies in the region of 150 cm⁻³·kWh⁻¹ for a diodelaser and 100 cm⁻³·kWh⁻¹ for a CO₂ laser.
 44. A method according toclaim 29 wherein the laser is selected from the group consisting of aCOIL, Nd:YAG, CO₂ and diode laser.
 45. A method according to claim 29wherein the laser beam is delivered by a fiber optic cable.
 46. A methodaccording to claim 29 wherein the laser beam is delivered by a mobilebeam delivery system comprising a system of reflecting mirrors.
 47. Anapparatus for cutting thick sections of cement-based materials in whichmaterial is made molten and allowed to solidify, the apparatuscomprising a means for mutually traversing a surface to be cut with anunfocused laser beam at a power density sufficient to produce a depth ofmolten material having a maximum depth of 10 mm at each traverse; ameans for breaking the solidified material into particles; and a suctionmeans for removing the particles.
 48. An apparatus according to claim 47wherein the means for breaking re-solidified material comprises apercussive member for crushing the material.
 49. An apparatus accordingto claim 48 wherein the percussive member is hollow and crushed materialis removed through the member by suction means.
 50. An apparatusaccording to claim 47 wherein the laser beam is substantially parallel.51. An apparatus according to claim 47 wherein the laser beam has acircular or rectangular cross section.